B.Sc

On space mission different roles are performed by different members. Major functions are Pilot, Flight Engineer, Mission Commander and Payload Specialists. During some mission a Mission Specialist may be on board. A Mission Specialist is assigned to a limited field of the mission, such as for medical or technical experiments. If you want to be a Mission specialist you will have achieve excellence in your scientific profession beside pilot training and experience. In India astronauts were inducted from Fighter branch of Indian Air Force.

try to score good score in GATE and opt for
Either MSc physics or Maths
it will be a better option

With PCM you are eligible for all three and many more. Here is the link to explore
https://engineering.shiksha.com/msc-colleges-in-india-categorypage-2-65-1-0-0-1-1-2-0-none-1-0

Australia will be a good option for you.

There are many good B.Sc. colleges across India, specify the region you are looking for

No u can't it needs PCM

After bsc you can apply for state and central govt jobs.All jobs that demand graduation.
Bsc and Msc jobs depend upon streams.
Else if you have bsc and msc plain you can be a teacher or a faculty of colleges. Rest is all about enhancing skills and getting experience.

Mtech can be pursued if you clear the GATE exam wid gud score. You may go ahead. Even masters in USA is gud option but fees varies around 40lacs for 2yrs.